<template>
  <ul class="list">
    <li
        class="item"
        v-for="item of letters"
        :key="item"
        :ref="item"
        @touchstart="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
        @click="handleLetterClick"
    >
        {{item}}
    </li>
  </ul>
</template>

<script>
export default {
  name: "CityAlphabet",
  props:{
    cities: Object
  },
  computed: {
    letters() {
      const letters = []
      for (let i in this.cities){
        letters.push(i)
      }
      return letters
    }
  },
  data (){
    return {
      touchStatus: false,
      startY: 0,
      timer: null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods:{
    handleLetterClick (e){
      this.$emit('change',e.target.innerText)//这里传递给父组件City一个letter
    },
    handleTouchStart (){
      this.touchStatus = true
    },
    handleTouchMove (e){
      if (this.touchStatus) {
        if(this.timer){
          clearTimeout(this.timer)
        }
        this.timer = setTimeout(() => {
          const touchY = e.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY)/20)
          if(index >= 0 && index < this.letters.length) {
            this.$emit('change',this.letters[index])
          }
        },16)//停滞16ms再加载，防止手指再拖动字母那一栏的的时候，太快
      }
    },
    handleTouchEnd (){
      this.touchStatus = false
    }
  }
}
</script>

<style lang="stylus" scoped>
  @import '~styles/varibles.styl'
  .list
    display: flex
    flex-direction: column
    justify-content: center
    position: absolute
    top: 1.58rem
    right: 0
    bottom: 0
    width: .4rem
    .item
      line-height: .4rem
      text-align: center
      color: $bgColor
</style>
